Tension-reducing random sprocket

ABSTRACT

A sprocket is provided having a plurality of teeth around its circumference. Adjacent teeth are separated by roots each having a root radius defined as the distance between the center of the sprocket and a point along the root closest to the sprocket center in a radial direction. In one aspect of the invention, the sprocket comprises at least two different root radii arranged in a pattern effective to redistribute tensions imparted to a chain by the sprocket at one or more predetermined sprocket orders.

FIELD OF THE INVENTION

The invention relates generally to a sprocket, and particularly to asprocket for reducing chain tensions.

BACKGROUND OF THE INVENTION

Chain and sprocket systems are often used in automotive engine systemsto transmit rotational forces between shafts. For example, a sprocket ona driven shaft may be connected via a chain to a sprocket on an idlershaft. In such a chain and sprocket system, rotation of the driven shaftand driven sprocket will cause the rotation of the idler shaft and idlersprocket via the chain. In an automotive engine system, sprockets on thecrankshaft may be used, for example, to drive one or more cam shaftsprockets.

The chains used in chain and sprocket systems typically comprise aplurality of intermeshing link plates connected with pins or rollers.The sprockets typically comprise a circular plate having a plurality ofteeth disposed around the circumference thereof. Located betweenadjacent teeth are roots having generally arcuate or semi-circularprofiles for receiving the pins or rollers of the chain. Each root has aroot radius, defined as the distance from the center of the sprocket toa point on the root closest to the sprocket center.

In a “straight” sprocket the root radii are all substantially equal.However, it has been found that as a chain rotates around a straightsprocket, audible sound frequencies creating undesirable noise are oftengenerated as the pins or rollers connecting the links of the chaincontact the sprocket teeth and impact the roots disposed betweenadjacent teeth of the sprockets.

The sound frequencies and volume of such noise typically variesdepending on the chain and sprocket designs, chain rotational speed, andother sound or noise sources in the operating environment. In the designof chain and sprocket systems, it can be desirable to reduce the noiselevels generated as the rollers of a chain engage roots of a sprocket.

“Random” sprockets have been developed to help reduce the radiated noiselevels generated by the engagement of the chains with the sprockets.Random sprockets may be characterized by having a plurality of differentroot radii. The different root radii can be arranged in a pattern aroundthe sprocket circumference to modulate the sound frequencies generatedby the engagement of the chain rollers or pins with sprocket teeth androots. By modulating these sound frequencies, the noise generated as thechain rotates around the sprocket may be reduced.

In addition to minimizing noise generated by engagement between a chainand sprocket, it is also desirable to reduce the tensions imparted tothe chain by the sprocket. Reduced chain tensions can be advantageousbecause they may result in decreased wear of the chain, thus increasingthe life cycle of the chain. Furthermore, reduced chain tensions mayalso result in less wear to the sprocket, thereby also increasing thelife cycle of the sprocket.

It has been observed in chain tension measurements that certain chaintensions in a particular system may vary on a periodic or repeatingbasis, which often can be correlated to tension inducing events. Forexample, in automotive timing chain systems, it has been observed fromchain tension measurements that the engagement and disengagement of eachsprocket tooth and/or root with the chain pins often results inrepeating tension changes. These chain tension changes may be correlatedwith potentially tension inducing events, such as the firing of pistoncylinders, transmission engagements, etc.

A useful approach to analyzing such tension events is to observe thenumber of events that occur relative to a reference time period, as wellas the amount of the tension change for each event. For example, in anautomotive timing chain system, one may observe the number or frequencyof tension changes in the chain relative to rotations of a sprocket or acrankshaft, as well as the magnitude of the tension change in the chain.

In such a system, for example, a tensioning event that occurs once pershaft or sprocket rotation may be considered a “first” order event, andan event occurring four times for each shaft or sprocket rotation may beconsidered a “fourth” order event. Depending on the system and therelative reference period, i.e., rotations of the crankshaft or thesprocket (or another reference), there may be multiple “orders” ofevents in a crankshaft or sprocket rotation in such a system originatingfrom one or more tension sources. Similarly, a particular order of thesprocket rotation may include or reflect the cumulative effect of morethan one tensioning event. As used herein, such orders of tensioningevents occurring during a sprocket (or crankshaft) rotation also may bereferred to as the orders of a sprocket (or crankshaft) and/or sprocketorders (or crankshaft orders).

In straight sprockets, measurable tensions typically are imparted to thechain at a sprocket order corresponding to the number of teeth on thesprocket, also known as the pitch order. Thus, in a sprocket withnineteen teeth, tensions would be imparted to the chain at thenineteenth or pitch order, i.e., nineteen times per revolution of thesprocket. Depending on the sprocket design, the order in a straightsprocket would typically occur at equal intervals relative to thesprocket rotation, with a generally equal tension change or amplitude.

Random sprockets, in contrast, typically have different tensioningcharacteristics when compared to straight sprockets due to the differentroot radii. As the chain rotates around the random sprocket, each of thedifferent root radii typically imparts a different tensioning event tothe chain. For instance, as a roller of the chain engages a root havinga first root radius, the chain may be imparted with a tension differentfrom when a roller of the chain engages a root having a second rootradius larger than the first root radius. Tension changes, in addition,may also be imparted to the chain by a random sprocket due to therelative positioning of the different root radii. A roller movingbetween adjacent roots having the same root radii may result indifferent chain tension changes than a roller moving between adjacentroots having different radii.

The change in chain tensions imparted by random sprockets may be furtheraccentuated when the sprocket has more than two different root radii. Ina random sprocket having first, second, and third successively largerroot radii, the tension imparted to the chain may be greater when achain roller moves from a root having a first root radii to a roothaving a third root radii than when a chain roller moves from a roothaving a first root radii to a root having a second root radii.

Thus, random sprockets designed principally for noise reduction oftencause increases in chain tensions and tension changes as compared to themaximum tensions imparted to the chain by straight sprockets. Forexample, a random sprocket design may reduce chain noise or chain whineby reducing the pitch order of the sprocket. However, reducing the pitchorder of a sprocket may result in redistributing or concentrating thetensional forces imparted to the chain by the sprocket over the lowerorders of the sprocket. This often results in increased chain tensionscorresponding to the lower orders of the random sprocket.

The increased chain tensions at the lower sprocket orders frequentlycause the overall maximum chain tension force exerted on the chain andsprocket to increase. As a consequence, a chain and sprocket systemsubjected to such tensions typically will experience greater wear andincreased opportunities for failure, as well as others adverse effects,due to the concentration of the tensional forces in the lower orders.

Accordingly, there remains a need for a sprocket design and method ofdesigning sprockets that incorporates the noise reduction properties ofrandom sprockets without the increased maximum chain tensions associatedtherewith. In addition, there is a need for a sprocket design and methodfor designing sprockets to provide the flexibility to shift tensionalforces to different sprocket (or other) orders to enhance theperformance, durability, and efficiency of a chain and sprocket systemor other comparable systems.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a sprocket is providedthat has a root pattern and root radii selected to concentrate themaximum chain tensions at a predetermined order or at multiplepredetermined orders relative to the sprocket rotation or anotherreference, such as, for example, the rotation of a crankshaft inautomatic timing chain applications. In this aspect, a sprocket isprovided that promotes reduced or controlled chain tensions and may alsosimultaneously reduce chain noise.

In another important aspect, the order or orders of the sprocket havingthe concentrated chain tensions may be chosen to at least partiallycancel or add to corresponding tensions imparted to the chain fromsources external to the sprocket. By coordinating the maximum tensionsimparted to the chain by the sprocket with the maximum or minimumtensions imparted to the chain by sources external to the sprocket, theoverall maximum tensions in the system may be reduced or redistributedin a beneficial way.

In another important aspect of the invention, a sprocket is providedthat has a random tooth pattern and root radii that are selected toconcentrate the maximum chain tensions at a predetermined order of thesprocket (or other reference). The sprocket comprises a plurality ofradially extending teeth arranged around the circumference of thesprocket. Roots are defined between adjacent teeth and have a generallyconcave or semi-circular profile for engagement with roller or pinconnecting links of the chain. The roots each have a root radiusmeasured from the center of the sprocket to a point along the rootclosest to the sprocket center in the radial direction.

In accordance with another aspect of the invention, the tooth pattern ofthe random sprocket may comprise two or more different root radiibetween adjacent teeth arranged in a predetermined pattern around thecircumference of the sprocket effective to reduce the sound frequenciesgenerated as the rollers of the chain engage the sprocket teeth androots, and the overall tension forces in the chain and sprocket system.In another important aspect of the invention, three different root radiimay be arranged in a pattern around the circumference of the sprocket.The use of three different root radii to vary the depth of the spacesbetween adjacent teeth may further reduce the sound frequencies andoverall tension forces exerted on the system.

The root radii pattern according to the sprocket of another aspect ofthe invention also may be selected to redistribute the first, second,third, and fourth order tensions imparted to the chain by the sprocketto a fourth order of the sprocket revolution. In yet another aspect ofthe invention, a sprocket is provided that reduces chain tensions andchain noise, as compared to typical random sprockets designedprincipally for noise reduction.

In still another aspect of the invention, the maximum tensions impartedto the chain by the sprocket may be concentrated at the fourth order ofthe sprocket rotation. Accordingly, a peak in the chain tension impartedby the sprocket occurs four times for every rotation of the sprocket.

According to another aspect of the invention, the sprocket may be usedin a chain and sprocket system where tension is imparted to the chainfrom sources external to the sprocket. When the maximum tensionsimparted to the chain from sources external to the sprocket occur atgenerally predetermined orders of the sprocket rotation, the arrangementof the sprocket root radii may be selected to at least partially offset,disperse, compensate for, or add to the tensions imparted to the chainoriginating from external sources.

In another aspect of the invention, a random sprocket may be comprise aplurality of different pitch radii, as measured from the center of thesprocket to a chain joint when the chain is seated in the sprocket. Theplurality of different pitch radii may be arranged in a patterneffective to distribute chain tensions at one or more predeterminedorders of the sprocket revolution, such as for silent chains whereinteeth of the chain contact teeth of the sprocket.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view illustrating a straight sprocketaccording to the prior art;

FIG. 2 is a side elevation view illustrating a random sprocket accordingto the prior art;

FIG. 3 is a side elevation view illustrating a random sprocket accordingto one aspect of the invention;

FIG. 4 is a detail view of section 4—4 of FIG. 3 showing pins of a chainin sprocket roots;

FIG. 5 is a graph comparing the maximum chain tensions occurring atorders relative to a sprocket revolution with the sprocket orders usingthe prior art random sprocket shown in FIG. 1;

FIG. 6 is a graph illustrating the maximum chain tensions occurring atselected orders relative to a sprocket revolution using the randomsprocket of the invention shown in FIG. 3;

FIG. 7 is a graph comparing the maximum chain tensions of the sprocketsof FIGS. 1–3 with the speed of an engine; and

FIG. 8 is a detail view of a sprocket showing the teeth of a chainbetween adjacent sprocket teeth.

DETAILED DESCRIPTION

The invention in one important aspect is embodied in a random sprocketfor use in an automotive chain and sprocket system, such as used in anengine timing system. In this aspect of the invention, the sprocket rootradii and patterns are selected, and are effective, to redistributechain tensions to one or more predetermined orders of the sprocketrevolution. In another important aspect, the random sprocket has aplurality of different root radii arranged in a pattern effective toreduce tensions imparted to the chain by the sprocket. In yet anotheraspect, the root radii and pattern is selected to reduce tensionsimportant to the chain and to reduce noise generated as the chaincontacts the sprocket.

FIG. 1 illustrates a typical prior art sprocket 10. The sprocket 10 hasnineteen radially extending teeth 12 disposed about its circumferencefor engaging links 82 of a chain 80, such as the chain 80 illustrated inFIG. 8. Such sprockets 10 may have a variety of sizes, and, for example,may have an outer radius of approximately 3.0915 cm, as measured fromthe center of the sprocket 10 to tips of the teeth 12.

Sprocket roots 14 are defined between adjacent teeth 12 for receivingpins or rollers 84 that connect the links 82 of the chain 80. The roots14 have a generally arcuate profile to facilitate engagement with thepins 84 of the chain. Each root 14 has a root radius RR, defined as thedistance from the center of the sprocket 10 to a point along the root 14closest to the sprocket center. In the illustrated sprocket 10, the rootradius RR is approximately 2.57685 cm, as measured from the center ofthe sprocket 10 to the innermost point along the root 14. The sprocket10 of FIG. 1 has all of its root radii RR equal to each other, and isgenerally known as a “straight” sprocket. Thus, the depths of each root12 are the same, as indicated with reference numeral 1, corresponding tothe first (and only) root radius RR for this type of sprocket 10.

Different tensioning events of the chain 80 may be repeated on aperiodic basis during each rotation of the sprocket 10. As mentionedabove, the number of times a given tensioning event is repeated in onerotation of the sprocket 10 may be referred to as an “order” relative tothe sprocket 10 rotation. For example, a tensioning event of the chain80 that occurs once during each rotation of the sprocket 10 may betermed a first order event, events occurring twice during one sprocketrevolution may be termed second order events, etc.

When the tension in the chain 80 is observed during operation of thesystem, increases in the tension of the chain 80 may occur at certainorders of the sprocket 10 revolution. In a straight sprocket, such asthe sprocket 10 of FIG. 1, the only significant peak in the chaintension may occur at the order of the sprocket 10 corresponding to thenumber of teeth 12 on the sprocket 10, also known as the pitch order asmentioned above.

Thus, a chain rotating about the sprocket 10, having nineteen teeth 12,will have a peak in the tension imparted to the chain 80 by the sprocketat the nineteenth order of the sprocket revolution, or nineteen timesfor every revolution of the sprocket 10. Peaks in the tension impartedto a chain 80 by a sprocket 10 may also be due to other factors besidesthe number of sprocket teeth 12. For example, a sprocket 10 that is notrotating about its exact center may impart a tension to the chain 80 atthe first sprocket order, or once for every rotation of the sprocket 10,due to the eccentric rotation of the sprocket 10.

In order to reduce noise generated by contact between pins or rollers 84of a chain 80, and roots 14 and teeth 12 of a sprocket 10, “random”sprockets have been developed with plurality of different root radii.For example, a random sprocket may have two different root radiiarranged in a predetermined pattern selected to decrease noise. A randomsprocket may also be designed to incorporate three different root radiiarranged in a predetermined pattern to further reduce noise generated byengagement of the chain 80 with the sprocket. The root radii may varybased on the particular system and sprocket design.

EXAMPLE 1

The random sprocket 20 illustrated in FIG. 2 is designed to reduce noisegenerated by engagement of the chain 80 with the sprocket 20. The randomsprocket 20 is similar to the straight sprocket 10 of FIG. 1, but hasthree different root radii R1, R2, and R3 and thus three different rootdepths 1–3. In the sprocket 20 illustrated in FIG. 2, the first rootradii R1 is approximately 2.54685 cm, the second root radii R2 isapproximately 2.57685 cm, and the third root radii R3 is approximately2.60685 cm, as measured from the center of the sprocket 20 to theinnermost points of the roots 24.

The root depths 1–3 are arranged in a pattern selected to modulate theengagement frequency between the pins 84 of the chain 80 and roots 24between adjacent teeth 22 of the sprocket 20 in order to reduce noisegeneration. As the pins 84 of the chain 80 move between adjacent roots24 of the sprocket 22, the radial position at which the pins 84 seatvaries between a maximum radius, a nominal radius, and a minimum radius.In the noise reducing sprocket 20 of FIG. 2, the pattern of root 24depths, beginning at the timing mark T, is 2, 2, 3, 3, 2, 1, 1, 2, 2, 3,2, 1, 1, 2, 1, 2, 1, 1, 1.

The sprocket 20 was mounted on a shaft and used to drive a chain 80. Atension measuring device was placed in contact with the chain 80 tomeasure the tension as the chain 80 was driven by the sprocket 20. FIG.5 includes a chart 40 comparing maximum chain tensions withcorresponding orders of the rotation of the shaft on which the randomsprocket 20 of FIG. 2 was mounted. Thus, in this example, the rotationalspeed of the sprocket 10 is the same as the rotational speed of theshaft, and the sprocket 20 therefore has the same relative order numbersas the shaft. If the sprocket under consideration is a driven sprocket,the rotational speed and orders relative to the shaft may differ fromthe example, depending upon the size of the driven sprocket and therelative number of rotations the driven sprocket makes compared to thedriving sprocket 20.

FIG. 5, in addition, includes a table 42 listing the maximum chaintensions corresponding to the first nineteen orders of the revolution ofthe shaft and random sprocket 20. As shown in chart 40 and the table 42,the greatest chain tensions occur at the sixteenth and nineteenthorders, but the lowest sprocket orders also have relatively highcorresponding maximum chain tensions. For example, the first, second,third, and fourth sprocket orders have corresponding chain tensionswhich are considerably greater than the chain tensions occurring at thefifth through thirteenth orders of the sprocket.

Thus, in the random sprocket 20 having three different root radiiarranged in pattern selected for noise reduction, illustrated in FIG. 2,the first, second, third, and fourth sprocket orders may impartrelatively large tensions to the chain 80 as compared to the remainingsprocket orders. This increase in chain tensions corresponding to lowersprocket orders may have the undesirable effect of increasing theoverall maximum chain tensions, creating an imbalance in chain tensions,and reducing the overall life of the chain and/or sprockets.

In order to reduce the chain tensions resulting from random sprocketssuch as in Example 1, the tensions imparted to the chain 80 by asprocket utilizing one aspect of the invention may be redistributedamong select sprocket orders or concentrated at a predetermined sprocketorder. In this aspect, a plurality of different root radii are used andthose root radii are arranged in one or more patterns that are effectiveto redistribute chain tensions occurring at one or more sprocket ordersto other sprocket orders. The root radii and patterns also may beselected to reduce chain noise or whine without the disadvantages ofrandom sprockets such as those discussed above and in Example 1.

In this aspect, the sprocket root radii are selected relative to amaximum radius and a minimum root radius as determined from the chainlink size and configuration; the chain connecting pin size and spacing;and/or the number of sprocket teeth, tooth configuration and sprocketssize. The root radii also may be selected relative to a nominal rootradius which typically is the mid-point between the maximum and minimumroot radii, and often is analogous to the root radii selected for asimilar straight sprocket.

The selection of varying root radii allows for the distribution of thepitch tensions generated by the chain to sprocket tooth/root contact. Itis believed that this is due to the contact of the chain pins (orequivalent chain elements) with the sprocket teeth/roots at differenttimes and at different tension levels as a result of the varying depthsof the sprocket roots.

The root radii further are arranged in a pattern that repeats around thesprocket circumference. This pattern typically includes one or more setsor multiple, non-uniform or random root radii. Each set typicallyincludes the same number of root radii having the same length andarranged in the same order. Different sets of root radii may have radiiof different lengths, number and arrangement.

The use of such patterns of sets of otherwise random root radii repeatedalong the circumference of the sprocket permits the shifting of thosetensions to specific sprocket orders (or other orders based on theapplicable reference). In doing so, the cumulative effect of shiftingthe tension forces permits the planned reduction or increase in theamount of chain tension is incorporated to the system by the sprocket atspecific sprocket orders (or other reference orders).

The selection of the patterns of non-uniform or random root radii, andthe lengths of those radii further permits the use of major and minorpatterns or sub-patterns of root radii. Such major and minor patternsare effective to redistribute the tensions imparted to the chain (andoverall system) to multiple sprocket orders (or other applicable orders)and at different magnitudes. This provides the additional flexibility inthe selection of the sprocket root radii and patterns to offset multipletension sources in the system and/or to balance the overall tensions onthe chain and sprocket regardless of other sources of the tensionalforces.

Moreover, the selection of such root radii and patterns permits thedesign of a sprocket and chain system that provides reduced levels ofchain noise with the proper selection of the root radii within a set ofradii, without unduly increasing overall chain or system tensions.Similarly, such a design is capable of providing reduced chain noiselife and durability of the chain and a balance of tensional forcesextending the sprocket system.

The following Example 2 illustrates several important aspects of theinvention. Other variations and applications of the invention also arediscussed below.

EXAMPLE 2

FIG. 3 illustrates a sprocket 30 according to an aspect of the inventionwherein a random sprocket 30 is provided for both redistributing chaintensions at a predetermined sprocket orders and reducing noise generatedby engagement of the chain 80 with the sprocket 30. Similar to thestraight sprocket 10 of FIG. 1 and the random sprocket 20 designedprincipally for noise reduction of FIG. 2, the sprocket 30 has aplurality of radially extending teeth 32 disposed about itscircumference for engaging the pins 84 of the chain 80. Roots 34 aredefined between adjacent teeth 32 for receiving the pins 84 that connectthe links 82 of the chain 80.

The sprocket 30 of FIG. 3 has a maximum root radius R3, a nominal rootradius R2, and a minimum root radius R1. As mentioned above, the maximumand minimum root radii are dependent on the link size and pin spacing,the shape of the sprocket teeth, etc. The root pattern of the sprocket30 of FIG. 3 is different from the root pattern of the sprocket 20 ofFIG. 2.

As illustrated in FIG. 3 was prepared with root radii R1, R2, and R3 ofapproximately 2.54685 cm, 2.57685 cm, and approximately 2.60685 cm,respectively. The pattern of root depths, beginning at the timing markT, is 2, 3, 3, 2, 1, 2, 3, 3, 2, 1, 2, 3, 3, 2, 1, 2, 3, 3, 2. The rootradii pattern of the sprocket 30 contains a sequence, i.e., 2, 3, 3, 2,1, that is substantially repeated four times around the circumference ofthe sprocket 30.

Thus, the use of a random pattern of root radii grouped in sets of radiiin this example provide a repeating pattern may be used to effectivelyshift and concentrate the lower order tensions of the chain 80 at thefourth order of the sprocket 30, thereby reducing the overall maximumtensions imparted to the chain 80 by the sprocket 30. Such use of acombination of random root radii and repeating root radii patterns thatshifts tensions imparted to the chain 80 by the sprocket 30 alsoprovides benefits in that the overall maximum chain tensions may bereduced, while also reducing chain noise or whine.

As illustrated in FIG. 4, showing a detailed view of a portion of thesprocket 30 of FIG. 3, the sprocket has three different root radii R1,R2, and R3, and thus three different root depths 1–3. FIG. 4 also showsarcs A1, A2, and A3 through the centers of chain pins 84 that correspondto the maximum seating radius R3, the minimum seating radius R1 and thenominal seating radius R2. The different root depths 1–3 are arranged ina pattern about the circumference of the sprocket 30 that is selected toconcentrate the tensions imparted to the chain 80 by the sprocket 30 ata predetermined sprocket order.

The arrangement of the root radii may be selected by substantiallyrepeating the root radii pattern a number of times equal to the sprocketorder at which it is desired to concentrate the chain tensions. Forinstance, to concentrate the tensions imparted to the chain 80 by thesprocket 30 of the invention at the fourth sprocket order, thearrangement of the root radii may comprise a pattern that substantiallyrepeats four times around the sprocket 30.

FIG. 6 includes a chart 60, similar to the chart of FIG. 5, comparingthe expected maximum chain tensions with corresponding orders of therevolution of a shaft on which the random sprocket 30 is mounted. Thus,the orders of the sprocket 30 directly correspond to the orders of theshaft. In a table 62 of FIG. 6, the maximum chain tensions expected tocorrespond to the first nineteen orders of the sprocket revolution arelisted.

As shown in the chart 60 and the table 62, the sprocket 30 of FIG. 3 isexpected to have reduced chain tensions corresponding to the lowersprocket orders, as compared to the lower orders of the sprocket 20 ofFIG. 2, as shown in chart 52 of FIG. 5. Unlike the sprocket 20 of FIG. 2designed only for noise reduction, the sprocket 30 of FIG. 3 isconfigured to concentrate the maximum chain tensions at the fourth andnineteenth orders of the sprocket revolution, while reducing the maximumchain tensions corresponding to the lower sprocket orders.

For example, the first, second, and third sprocket orders for thesprocket 30 of FIG. 3 are expected to have corresponding chain tensionsreduced by about 76%, 73%, and 30%, respectively, as compared to thesprocket 20 of FIG. 2. The sprocket 30 redistributes those tensions fromthe lower orders to the fourth order, which is expected to have acorresponding maximum chain tension nearly 400% larger than the randomsprocket 20 of FIG. 2, designed principally for noise reduction.

As mentioned above, the random and repeating root radii pattern canprovide the benefit of reducing the overall maximum tensions imparted tothe chain 80 by the sprocket 30, while also reducing noise generated bycontact between the sprocket 30 and the chain 80. The expected overallmaximum tension reducing effects of the random sprocket 30 of theinvention are illustrated in FIG. 7. The maximum tensions expected to beimparted to a chain by the sprockets 10, 20, and 30 of FIGS. 1–3 arecompared with corresponding engine speeds in FIG. 7.

As illustrated in FIG. 7, the straight sprocket 10 of FIG. 1 impartssignificantly lower maximum tensions to the chain 80 throughout thevarious engine speeds relative to a random sprocket 20 designed only fornoise reduction. In particular, it is expected that the maximum tensionsimparted to the chain 80 by the random sprocket 20, designed principallyfor noise reduction, are higher near engine speeds of 4000 rpm, whilethe straight sprocket 10 would impart much lower maximum tensions to thechain for the same engine speed.

The maximum tensions imparted to the chain 80 by the random sprocket 30designed for both noise reduction and reduced maximum chain tensions areexpected to be significantly lower than for the random sprocket 20designed principally to reduce noise. In fact, the tension reducingsprocket 30 may impart comparable, and in some instances, lower maximumtensions to the chain 80 than the straight sprocket 10 at engine speedsreflected in FIG. 7. Thus, FIG. 7 illustrates that the improved randomsprocket design 30 of the invention is expected to provide for reductionof maximum overall chain tensions, an effect that is not available withprior random sprocket designs.

Although the fourth order was selected in the aspect of the inventionillustrated in FIG. 3, chain tensions may also be concentrated at otherorders of the sprocket revolution. For example, a root radii pattern maybe selected that is effective to concentrate chain tensions at the thirdorder of the sprocket revolution. Such a pattern may include a rootradii sequence that is substantially repeated three times around thecircumference of the sprocket. For example, a root depth pattern forconcentrating chain tensions at the third sprocket order may be 1, 2, 3,3, 3, 2, 1, 2, 3, 3, 3, 2, 1, 2, 3, 3, 3, 2, 1, where a root depthpattern, i.e., 1, 2, 3, 3, 2, 1, is substantially repeated three timesfor each revolution of the sprocket.

In addition, the tensions imparted to the chain 80 by the sprocket maybe concentrated at more than one sprocket order. For example, a rootradii pattern may be selected that has a major root radii sequencerepeating twice for each revolution of the sprocket and a minor sequencethat repeats twice within each major sequence. Thus, in this aspect ofthe invention, the major and minor radii are provided by having theminor pattern repeating within the major repeating pattern. A benefit ofhaving both major and minor repeating patterns is the ability to furtherredistribute the sprocket orders at which tensions imparted to the chain80 by the sprocket occur.

Thus, for every revolution of a sprocket having such a pattern, themajor root radii sequence may impart two tensioning events, while theminor root radii sequence may impart four tensioning events. Thetensioning events imparted by the minor root radii sequence may be of alesser magnitude than the tensioning events imparted by the major rootradii sequence.

Chain tensions, in addition, may be imparted to the chain 80 by variousparts of the automotive engine system external to the sprockets, such asthe shaft, the pistons, and/or chain tensioners. Chain tensions furthermay vary according to the operating temperature of the system. Forexample, when the ambient temperature of the engine decreases, the chain80 may tend to cool and then contract, resulting in an increased chaintension. Conversely, when the ambient temperature of the engineincrease, the chain 80 may tent to expand, thereby decreasing thetension in the chain 80. These external sources may impart tensionevents to the chain 80 in addition to those imparted to the chain 80 bythe sprockets 20 and 30 of the above examples. These external tensioningevents may occur at intervals that correspond to orders of the sprocketrevolution.

In order to reduce overall chain tensions in the chain and sprocketsystem, the tensions imparted to the chain 80 by the improved random andrepeating root radii pattern of the invention, such as those of sprocket30, may be selected to at least partially offset tensions imposed on thechain 80 by such sources external to the sprocket 30 and chain 80. Inone aspect of the invention, the orders of the sprocket revolutioncorresponding to peaks in the chain tension due to external sources, aswell as those due to the sprocket 30, are determined. The sprocket 30 isthen configured to concentrate chain tensions at a sprocket order atwhich the chain tensions due to external sources are at a minimum. Thisprovides the potential to reduce the overall tensions in the chain 80,such as may occur if both the chain tension due to the sprocket 30 andthe chain tension due to external sources are at their maximums.

In another aspect of the invention, the sprocket 30 is configured toimpart maximum tensions to the chain 80 at orders that add to tensionsimparted by external sources. This provides the ability to concentratethe maximum chain tensions due to both the sprocket 30 and the externalsources at a predetermined sprocket order.

For example, when the external tensions occur four times for everyrotation of the sprocket 30, the root radii of the sprocket 30 may bearranged to concentrate the maximum tensions imparted to the chain 80 bythe sprocket 30 at sprocket orders phased to at least partially cancelthe external tensions imparted to the chain. In this manner, theexternal tensions in the chain 80 may be at least partially offset bythe sprocket tensions in the chain 80 to reduce the overall tension inthe chain 80 and increase the life cycle of both the chain 80 and thesprocket 30.

In another aspect of the invention, the sprocket 30 is configured toimpart increased chain tensions at one or more predetermined orders togenerate low order tension fluctuations. The low order fluctuations maybe used to help actuate a variable cam timing unit.

FIG. 8 illustrates a sprocket 100 according to an aspect of theinvention for use with a silent chain 90. The silent chain 90 comprisesa plurality of link plates 92, each having one or more teeth 96,pivotable relative to each other about joints 94. As the silent chain 90rotates around the sprocket 100, the teeth 96 of the chain 90 engageteeth 102 of the sprocket 100. The sprocket 100 has three differentpitch radii PR1, PR2, and PR3, as measured from the center of thesprocket 100 to joints 94 between link plates 92 having teeth 96 seatedbetween teeth 102 of the sprocket 100. FIG. 8 illustrates arcs PA1, PA2,and PA3 through the centers of chain joints 94 that correspond to thepitch radii R1, R2 and R3. The pitch radii PR1, PR2, and PR3 arearranged in a pattern effective to distribute tensions imparted to thechain 90 by the sprocket 100 at one or more predetermined orders of therevolution of the sprocket 100.

From the foregoing, it will be appreciated that the invention provides amethod and apparatus for reducing noise generated by the engagementbetween a chain and a sprocket, while reducing the tensions imparted tothe chain by the sprocket. While the figures are illustrative of aspectsof the invention, the invention is not limited to the aspectsillustrated in the figures. Furthermore, the invention is not limited tothe aspects described herein above or to any particular aspects.

1. A chain and sprocket drive system comprising: a chain having aplurality of pairs of links being interconnected by pins; one or moregenerally circular sprockets which operate at varying speeds and thechain having external tensions imparted to it originating from sourcesother than the sprocket, the sprockets having a plurality of teethspaced about their periphery, the sprockets having roots located betweenpairs of adjacent teeth for receiving the chain pins; each of the rootshaving a root radius extending between the center of the sprocket and apoint along the root closest to the sprocket center in a radialdirection; at least one of the roots having a first root radius, and atleast one of the roots having a second root radius, at least one of theroots having a third root radius, the second root radius being less thanthe first root radius, the third root radius being less than the secondroot radius; and the first, second, and third root radii arranged in apattern which continually repeats itself around the periphery of thesprocket, the repeating pattern effective to redistribute the externaltensions imparted to the chain, reducing maximum tension forces exertedon the chain during operation of the system relative to the maximumtension forces in the system where the sprocket is a straight sprocket.2. The chain and sprocket system according to claim 1, wherein thepattern substantially repeats three times.
 3. The chain and sprocketsystem according to claim 2 wherein the root radii are arranged in amajor pattern and a minor pattern.
 4. The chain and sprocket systemaccording to claim 1 wherein the root radii are arranged in a patternthat also reduces noise produced by the interaction of said chain andsaid sprocket.
 5. The chain and sprocket drive system according to claim1, wherein the first, second, and third root radii are arranged in apattern that substantially repeats four times around the sprocket.
 6. Asprocket comprising: a plurality of teeth disposed along a circumferenceof the sprocket, adjacent teeth having roots therebetween, each of theroots having a root radius defined as the distance between the center ofthe sprocket and a point along the root closest to the sprocket centerin a radial direction, the sprocket teeth and roots disposed to receiveand engage a circular loop of chain which operates at variable speeds;and at least three different root radii, including a first root radius,a second root radius and a third root radius, the second radius lessthan the first and the third less than the second, the at least threedifferent root radii arranged in a sequence which uninterruptedly andcontinually repeats itself around the sprocket for distributing thetensions imparted to the chain by the sprocket to one or morepreselected orders relative to the rotation of the sprocket, thedistributed tensions effective to offset tensions imparted to the chainby forces external to the sprocket reducing maximum tension forcesexerted on the chain relative to maximum tension forces in the systemwhere the sprocket is a straight sprocket.
 7. The sprocket according toclaim 6 wherein the root radii are arranged in a plurality of sequences,at least one of which is major sequence and at least one of which is aminor sequence.
 8. The sprocket according to claim 6 wherein thesequence of root radii also is effective to reduce the noise generatedby the interaction of the sprocket and a chain.
 9. The sprocketaccording to claim 6 wherein a preselected order comprises a fourthorder.
 10. The sprocket according to claim 6 wherein the sequencerepeats three times.
 11. The sprocket according to claim 6 wherein thesequence substantially repeats four times around the sprocket.
 12. Amethod of distributing tensions imparted to a chain and sprocket systemoperating at variable speeds, the method comprising: providing asprocket having a plurality of teeth separated by roots; providing eachroot with a root radius extending between the center of the sprocket anda point along the root closest to the sprocket center in a radialdirection; providing at least three different root radii; and arrangingthe different root radii in a pattern which continually repeats itselfat least two times for distributing the tensions imparted to the chainand sprocket system reducing maximum tension forces exerted on the chainrelative to maximum tension forces in the system where the sprocket is astraight sprocket.
 13. The method according to claim 12, wherein thepattern repeats three times.
 14. The method according to claim 12wherein a plurality of root radii patterns are selected, at least one amajor pattern and at least one a minor pattern.
 15. A method accordingto claim 12 comprising selecting the root radii pattern effective alsoto reduce the noise generated by the interaction of the chain with thesprocket.
 16. The method according to claim 12, comprising concentratingthe tensions imparted to the chain by the sprocket at a fourth sprocketorder.
 17. The method according to claim 12, comprising selecting theroot radii pattern effective to at least partially offset tensionsimparted to the chain by sources other than the sprocket to balance theoverall tension force imparted to the system by all tension sources. 18.The method according to claim 12 wherein the pattern repeats four times.19. An automotive timing system comprising: a chain which operates atvariable speeds, the chain having a plurality of pairs of links beinginterconnected by pins; and a generally circular sprocket mounted on acam shaft having a plurality of teeth spaced about the periphery, thesprocket having roots located between pairs of adjacent teeth forreceiving the chain pins, each of the roots having a root radiusextending between the center of the sprocket and a point along the rootclosest to the sprocket center in a radial direction, at least one ofthe roots having a first root radius, at least one of the roots having asecond root radius, and at least one of the roots having a third rootradius, the second root radius being less than the first root radius andthe third root radius being less than the second root radius, and thefirst, second, and third root radii arranged in a sequence whichuninterruptedly and continually repeats itself at least two times andwhich maintains the distance between the chain pins substantiallyconstant while the chain is engaged around the sprocket and effective toredistribute tensions imparted to the chain reducing maximum tensionforces exerted on the chain during operation of the system.
 20. Theautomotive timing system according to claim 19, wherein the sequencecomprises second, third, third, second, first, second, third, third,second, first, second, third, third, second, first, second, third,third, and second root radii.
 21. The automotive timing system accordingto claim 19 wherein the root radii sequence is effective also to reducethe noise generated by the interaction of the chain and the sprocket.22. The automotive timing system according to claim 19, wherein thepattern comprises a sequence of second, third, third, second, first,second, third, third, second, first, second, third, third, second,first, second, third, third, and second root radii.
 23. An automotivedrive system comprising: a chain subject to tension loading sourcesexternal to the drive chain, the chain having a plurality of links, eachlink formed of two or more plates interconnected by pins, each pinhaving a central longitudinal axis, and the links providing contactsurfaces; the chain traveling in a loop about at least one sprocket indriving engagement with the chain and at least one sprocket in drivenengagement with the chain, each sprocket having a central axis ofrotation and plurality of surfaces spaced about the periphery of thesprocket disposed to engage the chain link contact surfaces; thesprocket engagement surfaces spaced a distance from the sprocket centralaxis to position the chain at a pitch radius defined by the distancebetween the sprocket central axis and the pin axis of a chain linkengaged by the surfaces; and the engagement surfaces of at least one ofthe sprockets disposed to engage the chain at least at a first pitchradius, at least at a second pitch radius, and at least a third pitchradius, the first, second and third pitch radii being different andarranged in a pattern which continually repeats itself, with eachrotation of the sprocket and the pattern imparting tensions to the chainat one or more sprocket orders effective to reduce maximum chaintensions during operation of the system relative to maximum chaintensions of the system where the sprocket is a straight sprocket. 24.The automotive drive system of claim 23 wherein the system operates atvariable speeds, the system speeds where chain tensions are generally ata maximum are the system resonance conditions; and the pattern of pitchradii are arranged to impart a maximum tension to the chain at saidsystem resonance speeds.
 25. The automotive drive system of claim 24wherein the first pitch radius is greater than the second pitch radiusand the second pitch radius is larger than the third pitch radius. 26.The automotive drive system of claim 25 wherein the pattern repeats atleast twice with each rotation of the sprocket.
 27. The automotive drivesystem of claim 23 wherein the pattern repeats at least three times witheach rotation of the sprocket.
 28. The automotive drive system of claim23 wherein the driving sprocket is rotated by an automotive powerplantat varying speeds; the powerplant operating at one or more speeds thatproduce substantially maximum chain tensions; and the pitch radiipatterns of the tension reducing sprockets are effective to reduce saidmaximum chain tensions relative to maximum chain tensions in a systemwhere the sprocket is a straight sprocket.
 29. The automotive drivesystem of claim 28 wherein the pitch radii pattern provided by eachtension reducing sprocket is effective to produce a maximum chaintension that is substantially equal to or less than the chain tension ina system where the sprocket is a straight sprocket through the normaloperating speed range of the powerplant.
 30. An automotive drive systemoperable at variable speeds comprising: a chain subject to tension loadsin a loop about at least one sprocket in driving engagement with thechain, and at least one sprocket in driven engagement with the chain,the system operating at one or more speeds where chain tensions reach apeak relative to chain tensions at other system speeds, the systemhaving tensions imparted from sources other than the chain and sprocket,the chain having a plurality of links formed of two or more platesinterconnected by pins, each pin having a central longitudinal axis andthe links providing chain contact surfaces, the at least one sprockethaving a central axis of rotation and a plurality of teeth end sprocketengagement surfaces between the teeth, the teeth and the sprocketengagement surfaces spaced about the periphery of the sprocket, thesprocket engagement surfaces disposed to engage the chain link contactsurfaces, the sprocket engagement surfaces spaced a distance from thesprocket central axis to dispose the chain at a pitch radius defined bythe distance between the sprocket central axis and the pin axis of achain link engaged by the surfaces; and the engagement surfaces of atleast one of the sprockets disposed to engage the chain to provide asequence of at least a minimum pitch radius, at least a maximum pitchradius, and at least a intermediate pitch radii therebetween, theengagement surfaces maintaining the distance between adjacent pin axesof links engaged with the sprocket substantially constant, and the pitchradii sequence uninterruptedly and continually repeating itself at leasttwice with each rotation of the sprocket for imparting tensions to thechain timed with respect to tension loads imparted to the system fromother sources effective to reduce maximum chain tensions at one or moreof the peak tension speeds relative to the maximum chain tensions atsaid peak tension speeds where the sprocket is a straight sprocket. 31.The automotive system of claim 30 wherein the sequence repeats at leasttwo times with each rotation of the sprocket.
 32. The automotive systemof claim 30 wherein the sequence repeats at least three times with eachrotation of the sprocket.
 33. The automotive system of claim 32 whereinan automotive power plant rotates the driving sprocket, the automotivepower plant imparting periodic tension loads on the chain, the systemreaching resonance conditions at powerplant speeds where the chaintensions reach their approximate maximum, and the sequence of pitchradii and the timing of the tensions provided by the pitch radiirelative to the power plant tension loads are effective to reducemaximum chain tensions during operation of the system at said resonanceconditions relative to the system where the sprocket is a straightsprocket operating at resonance conditions.
 34. The automotive system,of claim 30 wherein the sequence substantially repeats at least fourtimes with each rotation of the sprocket.
 35. A tension reducingsprocket for an automotive drive system having a continuous loop chainin driving engagement with a driving sprocket and a driven sprocket, thechain formed of two or more plates interconnected by pins, each pinhaving a central longitudinal axis, and the links providing contactsurfaces disposed to engage the sprocket, the sprocket comprising: asprocket body and a central rotational axis, the sprocket body providedwith engagement surfaces about its periphery, the engagement surfacesdisposed to receive the chain link contact surfaces in a drivingrelation, the engagement surfaces spaced a distance from the sprocketcentral axis to position the chain link received thereon at a pitchradius defined by the distance between the sprocket central axis and thechain link pin axis; and the engagement surfaces providing a repeatingsequence of at least, three different pitch radii which uninterruptedlyand continually repeats itself with each rotation of the sprocket, thesequence of the pitch radii effective to reduce maximum chain tensionsduring operation of the drive system relative to the system where thesprocket is a straight sprocket.
 36. The tension reducing sprocket ofclaim 35 wherein at least the first and second pitch radii are selectedand disposed to impose tension events on the chain timed with respect totorque loads imposed on the chain from other sources effective to reducethe maximum chain tensions during operation of the drive system relativeto a system where the sprocket is a straight sprocket.
 37. The tensionreducing sprocket of claim 35 wherein the engagement surfaces of thetension reducing sprocket are disposed to provide a repeating sequenceof pitch radii having a minimum pitch radius, a plurality of pitch radiiincreasing to a maximum pitch radius, and a plurality radii decreasingto the minimum pitch radius.
 38. A chain and sprocket drive systemcomprising: a chain having a plurality of pairs of links beinginterconnected by pins; one or more generally circular sprockets havinga plurality of teeth spaced about their periphery, the sprocket havingroots located between pairs of adjacent teeth for receiving the chainpins; each of the roots having a root radius extending between thecenter of the sprocket and a point along the root closest to thesprocket center in a radial direction; at least one of the roots havinga first root radius, and at least one of the roots having a second rootradius, at least one of the roots having a third root radius, the secondroot radius being less than the first root radius, the third root radiusbeing less than the second root radius; and the first, second and thirdroot radii arranged in a pattern that substantially continually repeatsitself four times around the sprocket, the pattern effective toredistribute tensions imparted to the chain, reducing maximum tensionforces exerted on the chain during operation of the system relative tothe maximum tension forces in the system where the sprocket is astraight sprocket.
 39. A sprocket comprising: a plurality of teethdisposed along a circumference of the sprocket, adjacent teeth havingroots therebetween, each of the roots having a root radius defined asthe distance between the center of the sprocket and a point along theroot closest to the sprocket center in a radial direction, the sprocketteeth and roots disposed to receive and engage a circular loop of chain;and at least three different root radii, the second radius being lessthan the first radius and the third radius being less than the secondradius, the at least three different root radii arranged in a patternaround the sprocket that substantially continually repeats itself fourtimes around the sprocket arranged in a pattern distributing thetensions imparted to the chain by the sprocket to one or morepreselected orders relative to the rotation of the sprocket, thedistributed tensions effective to offset tensions imparted to the chainby other tension sources reducing maximum tension forces exerted on thechain relative to maximum tension forces in the system where thesprocket is a straight sprocket.